It is often desired to map electrical properties over a surface of a body and to do so with fine resolution. The microelectronics industry uses semiconducting substrates (wafers) based on silicon, gallium arsenide, and other semiconductors and conducting layers deposited on these substrates. It is required that the uniformity of these substrates and layers be tightly controlled in order to achieve an acceptable yield of integrated circuits over a large wafer or substrate.
It is common to map the resistivity of a wafer in order to monitor wafer uniformity, both before processing and after ion implantation, layer growth, or deposition of conducting layers. The mapping may have the resolution of a few millimeters or even centimeters if only gradual variations across the wafer are anticipated. However, for some effects it is desired to have a technique that is more sensitive to electrical effects of smaller non-uniformities and defects rather than to smooth variations across the wafer. Probes of millimeter resolution are available that rely upon eddy-currents. Examples of such probes are gradient probes and differential probes, but in these examples their resolution lies above 1 mm.
In some applications, such as in the semiconductor fabrication industry, it is often desirable to control not only the resistivity, which is the product the carrier density and the mobility, but to separately control both of these parameters. Several contactless methods have been developed to make such concurrent measurements, for example: (1) microwave photoconductivity, in which the resistivity is measured through microwave reflection and the carrier density is varied by the level of optical illumination; and (2) Hall effect, measured by DC techniques in which a DC magnetic field is applied to the sample and the resultant magnetically induced voltage transverse to the applied voltage is measured.
Several methods have been proposed for mapping mobility and carrier density.
As Mittleman et al. has described in "Noncontact semiconductor wafer characterization with the terahertz Hall effect," Applied Physics Letters, vol. 71, no. 1, July 1997, pp. 16-18, one technique focuses a beam of terahertz radiation on a GaAs wafer and relies upon Faraday rotation to measure the mobility and carrier density. In Faraday rotation, the application of a magnetic field causes the polarization plane of the electromagnetic field to rotate. The effect can be considered in the context of transmission and reflection of electromagnetic waves from conductors to be a high-frequency Hall effect. The method is powerful, but its spatial resolution is diffraction limited and is presently about 250.mu.m.
Another technique has been described by Druon et al. in "Novel microwave device for nondestructive electrical characterization of semiconducting layers," Review of Scientific Instruments, vol. 61, 1990, pp. 3431-3434. This technique supplies 1GHz microwaves to two planar microstrip resonators scanned across the sample. The two resonators are perpendicularly arranged in a cross configuration. In the absence of a magnetic field, the two resonators are decoupled, but a magnetic field couples them through the Hall effect in the semiconductor sample. The technique can map mobility and carrier density with a resolution of several millimeters.
Another technique relies on an open microwave waveguide probe irradiating the sample. In the reflection mode, the same waveguide propagates the reflected radiation back to the detector. If the waveguide can support two orthogonal modes (for example, a circular waveguide), the rotated reflected component can be detected.
In the transmission mode, the receiving rectangular waveguide is rotated 90.degree. from the probing rectangular waveguide to pick up only the rotated transmitted component. For such probes, the resolution is limited to the dimensions of the waveguide, about .lambda./2.times..lambda./2, that is, no better than 1 mm.times.1 mm.
In U.S. patent application Ser. No. 08/526,659, filed Sep. 11, 1995 now issued as U.S. Pat. No. 5,701,018 and published as PCT Published Application WO 97/10514, incorporated herein by reference in its entirety, we have disclosed a microwave microscope that addresses at least some of these problems. It includes a microwave probe 10 illustrated isometrically in FIG. 1. A rectangular microwave waveguide 12 includes two side walls 14 of width a and two narrow side walls 16 of width b, all of which are highly conductive so as to support microwaves within the rectangular volume defined by the side walls 14, 16. The lateral wall dimensions are chosen to support microwaves within a predetermined frequency or wavelength band. For a given microwave wavelength .lambda..sub.0, there is a set of minimum dimensions lower than which the microwave radiation is not supported by the waveguide, and, as a result, the radiation quickly attenuates within the waveguide. There are no maximum dimensions, but the microwave radiation propagates with excessive loss when the waveguide size a, b is substantially larger than the microwave wavelength .lambda.. A circular or other cross-sectional waveguide will also propagate microwaves and thus be usable with the described probe.
A conductive foil end wall or faceplate 18 is joined to the waveguide 12 at its probe end and includes a located slit aperture 20 having a long dimension a' along the long waveguide side 14 and a short dimension b' along the short waveguide side 16. We now observe that the aperture need not be centrally located and that the slit need not be precisely parallel to the sidewalls of the waveguide 12 as long as its length is increased approximately by the arccosine of the angle. Instead of the conductive foil, a fairly thick dielectric wall can be used on which is coated a thin conductive coating.
The conductive coating may be deposited on either side of the dielectric wall 18 and electrically connected to the waveguide 12, and the aperture 20 may be formed in the conductive coating by chemical etching or laser ablation.
The dimensions a, b of the waveguide 14 are chosen to efficiently propagate microwave radiation of wavelength .lambda., as is well known in the technology of microwave waveguide transmission. It is further known that such an end wall with a narrow slit is transparent to microwave radiation if the dimensions are chosen such that ##EQU1## That is, under these conditions, the probe end efficiently propagates the probing radiation, but the near-field radiation field has a width (resolution) of the order of b' thereby providing microscopic resolution in the direction of the slit width b'. Generally the long slit length a' may be made slightly larger than the half microwave wavelength .lambda./2. An exemplary slit width b' that can be reliably machined is 100 .mu.m and is 10 .mu.m for vacuum deposited features, both appropriate for 80 GHz microwaves.
An example of a reflection-mode microwave microscope 30 using such a microwave probe 10 is illustrated schematically in FIG. 2. The slit 20 is cut into 20 .mu.m-thick aluminum foil, has dimensions of 1.5 mm by 100 .mu.m, and transmits at 80 GHz. The tested material 32, for example, a semiconductor wafer, is mounted on a movable X-Y stage 34 through an intermediate thick glass plate 36. It is alternatively possible to provide relative lateral movement between the slit 20 and the sample 32 by moving the waveguide 12. The slit 20 of the probe 10 is positioned to within a few micrometers from the front surface 37 of the wafer 32.
A source 38 of millimeter wave microwave radiation, for example at 80 GHz in E-band, provides the probing microwaves. An E-band millimeter-wave bridge is formed of a hybrid tee 40, an adjustable attenuator 42, a sliding short 44, and an E-H tuner 46 that matches the impedance of the slit antenna to that of the waveguide. A microwave detector 48 receives radiation from the bridge to thereby measure its imbalance, and the intensity is transmitted to a computer 50, which is also controlling a driver 52 scanning the X-Y stage 34 in a 2-dimensional scan. To increase sensitivity, a signal generator modulates the amplitude of the millimeter-wave source 38 and serves as a synchronizing signal for a lock-in amplifier receiving the output of the detector 48. The sensitivity can be further increased and the phase of the radiation can be measured by shifting the operating point of the detector 48 by varying the attenuator 42 and the sliding short 44.
The computer 50 outputs the detector signal registered with the X-Y position of the wafer 32. If the bias signal is in phase with the reflected signal, the detector voltage is sensitive to variations of the amplitude of the reflected signal, but, if the phase of the bias signal is in quadrature with the reflected signal, then the detector voltage is sensitive to phase variations. The amplitude gives information about the resistivity while the phase yields information about the dielectric constant.
Although the microwave microscope described above is effective at mapping the resistivity or dielectric constant, it is fundamentally a one-dimensional probe, i.e. along the narrow slit dimension b' although the lateral resolution can be improved by curving the slit in the length direction a The electrical polarization is restrained to be parallel to the short slit length both on incidence and on reflection. The probe 10 is relatively insensitive to polarization effects and to crystallographic anisotropies in the probed material. In particular, it cannot measure Hall mobilities, especially in conjunction with measuring resistivities.
It is desired to obtain such a microwave microscope that can measure polarization effects and anisotropies.
All these techniques also are directed to the measurement of characteristics varying spatially fairly slowly and are relatively insensitive to non-uniformities and defects which are substantially smaller than the short dimension of the slit. For example, the discontinuity presented by a grain of foreign material in a background matrix can be detected only if the electrical characteristic of the foreign grain is distinguishable from that of the background material.
It is further desired to obtain a microwave microscope that is more sensitive to very small though strong defects.